I think it's an historical thing, most audio inlets which might benefit from a signal level message have the [sig~] built in. I understand this might not always have been the case so some more old school PDers use it, and they remain in the help files.
I don't know if there's some minor technical benefit to using the [sig~] object, but I've still to find a need for it
> Date: Tue, 2 Feb 2010 20:51:45 -0800
> From: jancsika@yahoo.com
> To: pd-list@iem.at
> Subject: [PD] float to a signal inlet
>
> Hello,
> Taking acoustics~-help.pd as an example: is there any reason for
> the [sig~] object here? Doesn't the float from the gatom automatically
> get promoted to a signal?
>
> Thanks,
> Jonathan
>
>
>
>
>
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